The catalytic mechanism of 1-aminocyclopropane-1-carboxylic acid oxidase

It has been proposed that 1-aminocyclopropane-1-carboxylic acid (ACC) oxida se catalyzes the oxidation of ACC to ethylene via N-hydroxyl-ACC as an inte rmediate, However, due to its chemical instability the putative intermediat e has never been isolated. Here, we have shown that a purified recombinant ACC oxidase can utilize alpha -aminoisobutyric acid (AIB), an analog of ACC , as an alternative substrate, converting AIB into CO2, acetone, and ammoni a. We chemically synthesized the putative intermediate compound, N-hydroxyl -AIB (HAIB), and tested whether it serves as an intermediate in the oxidati on of AIB. When [1-C-14]AIB was incubated with ACC oxidase in the presence of excess unlabeled HAIB as a trap, no labeled HAIB was detected. By compar ing the acetone production rates employing HAIB and AIB as substrates, the conversion of HAIB to acetone was found to be much slower than that of usin g AIB as substrate. Based on these observations, we conclude that ACC oxida se does not catalyze via the N-hydroxylation of its amino acid substrate. A CC oxidase also catalyzes the oxidation of other amino acids, with preferen ce for the D-enantiomers, indicating a stereoselectivity of the enzyme. (C) 2001 Academic Press.